1. Field of the Invention
The present invention relates to a semiconductor device and its mounting method and, more particularly to a semiconductor integrated circuit device of vertical surface mounting type.
2. Description of the Related Art
High mounting density is required in mounting a semiconductor device, particularly an integrated circuit such as a memory, onto a printed board. To satisfy this requirement, the method for mounting the semiconductors has been changed from an insertion mounting to a surface mounting. As a result, there is no need that an insertion hole is formed in the printed board, and mounting density can be improved. Regarding the surface mounting, the so-called soldering reflow in which electric parts fixed to the printed board are passed through a heating furnace has been used. If reflow is used in the surface mounting of the semiconductor integrated circuit device of a multiple pin package, a lead pitch can be reduced, and higher packaging density can be obtained.
Moreover, regarding the package of the semiconductor such as a memory, a horizontal package represented by a conventional dual in-line package (DIP) is changed to a vertical package represented by a zig-zag in-line package (ZIP). As a result, the number of the semiconductor devices to be mounted per unit area of the printed board can be increased, and high density mounting can be made.
The electronic parts for surface mounting are structured not to fall down even if soldering is melted when reflow is performed. However, as shown in FIG. 20, in a case that a semiconductor device 901 of vertical mounting type is mounted on a printed board 903 via outer lead 904 extending vertically from the lower side of the body of semiconductor device 901, the center of gravity of the semiconductor device becomes high, and use of only soldering paste 905 even in an extended blob as shown in the side view of FIG. 20, provides insufficient force for fixing the semiconductor device. Moreover, since the semiconductor device of vertical mounting type is not structured to stand itself, the semiconductor device falls down if soldering melts when reflow is performed. Therefore, in mounting the semiconductor device of vertical mounting type on the printed board, it is difficult to use the surface mounting by reflow.
There have been proposed several methods for surface mounting the semiconductor device of vertical mounting type. For example, as shown in FIG. 21, an L-shaped outer lead 907 is made long in a plane direction of the printed board, and even the semiconductor device of vertical mounting type can stand itself. However, since the length of the outer lead 907 becomes longer, a printed wiring pattern 908 of the substrate surface is detoured around its course by the lead 907, which is extended right and left, as shown in the figure. As a result, the printed wiring pattern 908 of the substrate surface is complicated and enlarged. Moreover, the complication of pattern design, the rise of the manufacturing cost, and the enlargement of the substrate area are brought about. For the above reasons, such a printed wiring pattern 908 is insufficient for high packaging density. Furthermore, the broken lines of the both side portions have no relation with the circuit of the inner portion of the semiconductor integrated circuit device, and the arrangement of these lines only means that the lead line 907 for mounting the device on the substrate is avoided. This brings about the enlargement of the semiconductor integrated circuit device.
As shown in FIG. 22, a projection 909, which is longer than the length of the L-shaped outer lead 904 in the vertical direction, is formed on both sides of the package in a case that resin-sealing is performed. In this case, the semiconductor device is fixed as vertically standing till soldering is hardened. Due to this, since at least two through holes 911 per one semiconductor device must be formed in the printed board 903, mounting efficiency is lowered. Moreover, in the case that both-side mounting or multi-layer mounting circuit is used, there is a problem in that a circuit pattern must be enlarged so as to detour the front and back through holes 911. Furthermore, since material of the mounting substrate is different from that of sealing resin when reflow is performed, there is a problem in that a crack is generated in the mounting substrate. These problems cannot be solved even if the material of the projection is changed from epoxy resin to Fe-Ni, which is the same material as the lead frame.
As mentioned above, if the semiconductor device of vertical mounting type is surface mounted, there is a problem in that high density mounting, which is an object of surface mounting, cannot be realized.